DESCRIPTION (Verbatim From the Applicant's Abstract): The cerebellar circuitry develops through the coordinated outgrowth of the axons of the two principal classes of neurons, the granule cell and the Purkinje cell, and the formation of interconnections among them. The proposed research will study the genetic control of cerebellar neurite extension. To identify genes that function in the signal cascades required for axon outgrowth, we have cloned two mouse homologues of the C. elegans gene UNC51 that encodes a protein serine/threonine kinase that functions in axon formation. Preliminary experiments demonstrate that one of the murine homologues, Unc51.1 is expressed in cerebellar granule neurons and functions in granule cell neurite extension in vitro and in vivo. The proposed research will use a combination of in vitro primary neuron culture, molecular cloning, biochemistry and mouse molecular genetics to test the hypothesis that Unc51.1 and Unc51.2 have essential roles in axon formation in the developing cerebellar cortex. The Research Plan has three main aims. The first group of experiments will characterize the expression and function of Unc51.2 in developing and adult brain. The function of the gene in granule cells or Purkinje cells will be tested by expressing a kinase deficient form of the gene and assaying neurite outgrowth in vitro and in vivo. In a second group of experiments, we will use the yeast two-hybrid system to identify signaling pathways for murine Unc51.1 and Unc51.2. Finally, we will generate in vivo models of Unc51.1 and Unc51.2 to test for protein function by making null alleles of each of the genes using conventional gene targeting approach. In mice with null alleles, we will be able to examine the function of the genes in neurite extension in all of the CNS neurons in which the gene is expressed. In the BAC transgenic animals we can examine the effect of kinase-deficient neurons on the development of the other cerebellar neurons and glia.